The quality of an electrical waveform which is generated by an optical shaft angle encoder of the type described in U.S. Pat. No. 4,266,125, which is expressly incorporated herein by reference, is related to the degree of collimation of the light beams which are used to detect shaft rotation. The degree of collimation is, in turn, related to both the physical size of the emitters which are used to generate the light flux and to the focal length of the collimation lenses which are used to create collimated light beams from the light flux. Thus, for optimal performance, a point source is ideal as a collimated light source. However, the emitted light from a point source emitter is generally insufficient for use, and the emitter size needs to be increased. As the physical size of an emitter is increased in order to increase the total power of the emitted light, the shadows cast by an optical encoder, for example, spokes of a rotating code wheel, becomes increasingly blurred and indistinct. A disadvantageous result of the use of blurred shadows is that the electrical waveform generated by the optical encoder is degraded from an optimal pure triangle waveform to a rounded triangle waveform having a lower signal-to-noise ratio. The shadow-blurring effect of increased emitter size has been somewhat reduced in optical shaft angle encoders constructed according to the prior art by minimizing the size of the gap between the code wheel and the phase plate. This attempted solution has the major disadvantages of complicating the mechanical design of the optical shaft angle encoder while also increasing the risk of mechanical failure due to the necessary proximity of the phase plate to the rotating code wheel.
In a preferred embodiment of the present invention an optical shaft angle encoder utilizes slit emitters which are positioned so that the slit emitters are aligned with the collimation lenses, the code wheel spokes, and phase plate apertures. The slit emitters are configured to have a narrow width perpendicular to the radial axis of the code wheel and a length coincidental to a code wheel radial axis. The degree of light collimation, in the direction perpendicular to the code wheel radial axis, is thus dependent upon the width of the slit emitters. Therefore, degree of light collimation in this direction, viz., in an axis perpendicular to the length, shadow distinctness, and waveform clarity are all maximized at a given gap size and a given collimation lens focal length by minimizing the width of the slit emitters to uniaxially approximate light emission from a point source. At the same time, the total power of the light generated by the slit emitters can be increased, as needed, merely by increasing the length of the slit emitters without necessitating the use of increased emitter current densities or enlargement of the point source as required in the prior art.